The present invention generally relates ATM (asynchronous transfer mode) switching system having a dual ATM switch structure.
There is a limit on the bit rate of data transmission in a conventional switching system by the use of public lines. From this point of view, there has been considerable activity regarding the development of an ATM switching system which can transfer data at higher bit rates.
In an ATM switching system, data is divided into cells. One cell is the unit of data transmission. A header is included in each cell having data. The ATM switching system selects a data transmission route by means of hardware in accordance with the contents of the header. The ATM switching system can provide a large number of data transmission routes, which are realized by buffers.
Generally, two identical primary structural elements in a switching system are provided in order to realize high reliability. For the same purpose, it is desirable that the number of primary structural elements in even ATM switching systems be doubled. A conventional ATM switching system has a plurality of storing buffers arranged in a matrix, and each of the buffers is capable of storing a plurality of cells. Each buffer can always accept input cells successively and output cells successively. In other words, cells are successively input to each buffer, while each buffer also outputs cells successively. In a case where the ATM switching system has two identical ATM switches and these ATM switches are in service, it is necessary for the two in-service ATM switches to operate in the same way. That is, the two ATM switches must have the same operating state. In this case, one of the two ATM switches which are in service serves as a master ATM switch and the other ATM switch serves as a slave ATM switch.
Since the master ATM switch and the slave ATM switch operate in the same way, it is possible to switch the ATM switches from the master ATM switch (in a master mode) to the slave ATM switch (in a slave mode) when a failure occurs in the master ATM switch. The ATM switch in which a failure takes place is set to the out-of-service state and is investigated. After the failure in the ATM switch is eliminated, it is switched from the out-of-service state to the in-service state. It should be noted that at this time there is no cell in the ATM switch. That is, the contents of the ATM switch which will be set to the in-service state are different from those in the master ATM switch which is operating. Thus, even when the ATM switch is switched from the out-of-service state to the in-service state, it is impossible to set the ATM switch to the master ATM switch. As a result, it is desired to overcome the above-mentioned problem.